COMMUNICABLE DISEASE PREVENTION AND CONTROL
Seasonal Influenza Report for 2007 – 2008

St. Charles County Department of Community Health and the Environment
1650 Boone's Lick Road
St Charles, Missouri 63301
Prepared By: Alison Tucker
Epidemiologist
St. Charles County
Telephone: (636) 949-7400 extension 4226 or 4228
Fax: (636) 949-7414

Cases
There were 2106 cases of influenza¹ reported in St. Charles County during the 2007–2008 influenza season. Case reporting started in January, climbed quickly in Februray, peaked on week 5 (beginning of February), and decreased considerably by the end of February. In fact, over 75% of cases were reported in February 2008.

¹ Cases are defined as laboratory confirmed reports from hospitals, physician offices and long term care facilities. There is some reporter bias as not all laboratory confirmed cases are reported.

There were a total of 1044 Type A (50%), 278 Type B (13%) and 784 Unknown Type (37%) viruses. The graph below shows the percentage distribution of Influenza types by week. We saw a higher percentage of Type A in the beginning of the season and this changed by the end of February (around the same time cases started to decrease). After week 9 Type B cases were making up about 50% of the cases reported.

Vaccine
This next graph compares the last 5 influenza seasons in St. Charles County and clearly shows the extent of infection this past season. This seasons numbers of 2106 is almost tripled the mean of the previous 3 influenza seasons (Mean = 743 for 2004-05, 2005-06, 2006-07). The last year numbers were this high was the 2003-04 season (Graph 3).

According to the Centers of Disease Control and Prevention (CDC) part of the reason for this high number of cases is the mismatch of this season's influenza vaccine. A recent summary (CDC week 19: http://www.cdc.gov/flu/weekly/index.htm) of the 881 influenza viruses that were antigenic characterized shows a similarity between this season's circulating virus strains and this season's vaccine at only about 40%. A majority of the influenza A (H3N2) and the Influenza B viruses were not optimally matched. The last time there was a low match between the vaccine and circulating virus was 2003–04 season and if you look at Graph 3 we can see how this affected the influenza season (1291 cases in St. Charles County). In fact, since 1988 there has 4 seasons (1992–93, 1997–98, 2003–04, and 2007–08) when there was low crossNd reaction (http://www.cdc.gov/flu/about/qa/season.htm).

A summary of the 2007–08 influenza vaccine components and CDC influenza vaccine 2008–09 recommendations are seen in Table 1 below.

Even though there was a less than ideal match with the influenza vaccine this year, it is still important to be vaccinated for the following reasons:

• Influenza vaccination can still provide protection against influenza infection even when the vaccine strains are not ideally matched. It may result in reduced vaccine effectiveness against the variant viruses, but there is still enough protection to prevent or lessen illness severity and prevent flu–related complications.
• There are three virus strains in the influenza vaccine so even if the there is mismatch on one strain you can still be protected on the other 2 strains.

Demographics
Age
This graph shows influenza cases by age group in St. Charles County. Most reported cases are in the 05–14 and 25–49 age groups. In previous years the majority of cases were reported in the 5–14 age group presumably because of reporter bias among pediatric offices. It is a little unusual to see the highest number in the 25–49 age group compared to previous years, and part of the explanation could be that this age group is less likely to receive the flu vaccine.

In contrast to the above graph, the graph below shows influenza rates per 10,000 people for each age groups. This gives us a better idea of the age groups that are most reporting influenza. Rates are based on 2006 population estimate from the US Census Bureau. From this graph we see that the highest rates are in the 0–5 and 5–14 age groups which further supports our hypothesis that there may be reporter bias among pediatric offices.

Reporters
Most of the cases were reported by physician offices, followed by hospitals, urgent care, and long term care facilities.

Reporter	Number of Reports N(%)
Physician Offices	1094 (51.9%)
Hospitals	652 (31.0%)
Urgent Care	301 (14.3%)
Unknown	50 (2.4%)
Long Term Care	9 (0.4%)
Total	2106

Testing
All testing performed on reported St. Charles County cases this season was rapid diagnostic tests. It is important to note that the median senitivities of rapid tests are 70–75%. Therefore ~30% of the reported cases (631.8) are false positives. This does not negatively affect our analysis since rapid tests are useful in determine extend of influenza outbreaks quickly.

Geography
The next graph displays rates per 100,000 people per zip code. There were higher than normal rates in the 63348 and 63385 zip codes. Part of the explanation could be reporter bias. Another reason is that both those zip codes have experienced rapid growth in the past couple of years and since rates are based on 2002 census data it is inflating the rates.

Emergency Room Illness Surveillance
This is data taken from Missouri ESSENCE which is syndromic surveillance system looking at chief complaint data from hospital emergency rooms in Missouri. This graph looks at influenza–like ilness (ILI) in Eastern Missouri (Region C) comparing the curent season to last year 06–07 season.